Polyolefins and polyolefin compositions are typically used in products that are subjected to elevated temperatures which accelerate the rate of oxidation degradation in the polyolefin composition, commonly referred to as "heat aging", where long term performance is desired or required. Thus, the ability of polyolefins and compositions thereof to retain their original properties, such as elongation and tensile strength, at elevated temperatures during the required or desired term of performance is important.
Various antioxidant compounds or combinations thereof have been found to have a retarding or stabilizing effect on the "heat aging" of polyolefins and polyolefins compositions and therefore have been widely used. For example, U.S. Pat. No. 2,997,456 discloses stabilization of polyolefins against thermal degradation by incorporating metal salts of mercapto, selenide or telluride benzimidazole compounds into the polyolefins.
U.S. Pat. Nos. 4,260,661 and 4,824,883 disclose using an antioxidant system comprising a combination of a zinc salt of a mercaptoimidazole with a sterically hindered di-tertiary butyl phenol in polyolefin polymers and compounds thereof, such as polyethylene and copolymers of ethylene and other polymerizable materials.
Japanese Patent 58-122943 discloses a synergistic combination of antioxidants, a phenol, a 1,2-dihydroquinoline and an organic zinc salt, incorporated in polyolefins to achieve better heat stability.
While the polyolefin compositions of the above-mentioned prior art give improved heating aging properties by incorporating antioxidants or combinations thereof, the percent retention of these original properties is still low.